1. Technical Field
The present disclosure relates to a method of manufacturing an electronic device realized on plastic substrates, such as active matrix display devices or photovoltaic panels or sensors network.
The disclosure refers, in particular, but not exclusively, to a method of manufacturing an electronic device comprising a plastic substrate fixed to a rigid carrier, and the following description is made with reference to this field of application just for explanation convenience.
2. Description of the Related Art
Recently, interest in plastic devices for displays, photovoltaic panels and sensor networks has increased even further, due to the low cost, light weight, flexibility, and resistance to breaking of such devices. Despite this interest, there is still a need for a plausible manufacturing route for mass production of this kind of devices.
A number of different ways have been reported for the manufacturing of electronic devices on plastic substrates; however, all these manufacturing ways have some drawbacks mainly due to:                the difficulty of setting-up processes compatible with plastics such as by using low temperatures and non-aggressive chemicals;        managing of flexible, light and sometimes transparent devices using conventional equipments generally suitable for processing rigid substrates.        
It is well known that a possibility for manufacturing flexible devices using conventional equipment is to process a plastic substrate fixed over a rigid carrier substrate. The rigid carrier substrate is then released from the plastic substrate after having formed the circuit layouts over the plastic substrate. This enables to employ substantially conventional substrate handling and cell making processes. This known technique is called “temporary bonding” because it involves fixing the plastic substrate on the rigid substrate, also called the carrier, for a limited time period and in removing the plastic substrate from the carrier at the end of the manufacturing process.
As an example, FIGS. 1A-1C show the steps of a manufacturing process of an electronic device having a plastic substrate, according to the prior art. In particular, FIG. 1A shows a rigid carrier 1 and a plastic foil 2 that can be cut, modeled and bonded on the carrier 1, thus covering it as a plastic substrate 3 as shown in FIG. 1B. FIG. 1C shows an electronic device 4 manufactured on the plastic substrate 3 and released from the carrier 1.
Several methods are used to release the carrier from the plastic substrate at the end of the manufacturing process, depending on the plastic materials being used. In particular, a first type of plastic materials, such as polyethylene naphthalate (PEN), polyethylene terephthalate (PET), Arilite, Kapton, etc., can be laminated on the carrier. A second type of plastic materials can be obtained from liquid precursors, such as polyimide, silicon pastes, etc., which are deposited on the carrier, as they are or covered by either a metal or oxide or nitride layer, and then cured at high temperatures.
For the first type of plastic materials, a first known manufacturing process involves the steps of:                laminating a double-sided tape on the carrier; laminating the plastic materials on the double-sided tape;        realizing the electronic device on the plastic material;        releasing the carrier from the plastic substrate by putting it over a hot plate to increase its temperature.        
Even if the double-sided tapes are very resistant to chemical agents being used in manufacturing processes and even if they do not release any impurity both on the plastic substrate and on the carrier, allowing the re-use of the carrier, they are however not very resistant to temperatures greater than 150° C., whereat an anticipated de-bonding of the carrier occurs.
A second known manufacturing process involves the steps of:                depositing a liquid glue on the carrier;        laminating the plastic materials on the carrier;        realizing the electronic device on the plastic material;        releasing the carrier by mechanically peeling the plastic substrate or chemically removing the glue.        
Although the glue is more resistant to high temperatures, up to 200° C., it could be easily damaged by the chemical substances used in device manufacturing processes. Moreover, the de-bonding may cause damaging of the electronic device already formed, either in case of a mechanical de-bonding or a chemical attack.
A third known manufacturing process involves the steps of:                providing an electrostatic carrier comprising a pair of electrodes on which a dielectric material is deposited;        providing a plastic substrate adhering to the carrier due to the electrostatic field formed on it by an external power supply;        realizing the electronic device on the plastic material;        moving the device away from the carrier.        
The advantage of this technique is to avoid the surface contamination caused by glue or by adhesive tapes, so that the carrier can be re-used for producing standard devices in semiconductor industries. Nevertheless, the electrostatic field exponentially decreases with time and is very sensitive to thermal processes, causing the anticipated de-bonding of the carrier.
For the second type of plastic materials, a first known manufacturing process involves the steps of:                providing a carrier with a superficial sacrificial layer;        spinning a liquid precursor on the carrier, in such a way to adhere to the sacrificial layer;        curing the precursor to form a thin film;        realizing the electronic device on the thin film;        removing the carrier by a wet etch or of an anodic dissolution.        
The biggest problem of this type of de-bonding is that the carrier cannot be re-used. Moreover, in case of wet etch, the diffusion velocity of the removing solution is very limited, so that, for devices having an area of cm2, the duration period of the wet etch could cause damaging of the devices. This is way the wet etch is more applicable for small devices and for carriers having a big area exposed to the removing solution. Similarly, the de-bonding by anodic dissolution involves a more rapid corrosion of the metals of the carrier exposed to the anodic solution as much as greater is the area of the exposed metals. In addition, this type of de-bonding cannot be used if the plastic substrate covers completely the sacrificial layer and it requests the use of a dedicated equipment to control the parameters of the dissolution process.
There is another known manufacturing process, specifically used for liquid photosensitive precursors, such as polyimide, and for transparent carriers involving the steps of:                spinning a liquid precursor on the carrier;        curing the precursor to form a thin film;        realizing the electronic device on the thin film;        de-bonding the carrier by exposing the thin film to a laser having a specific wavelength, such as 308 nm (Xe—Cl) or 193 nm (ArF excimer laser), in such a way to ablate a sub-micrometric layer of the thin film which remains bonded to the carrier and to have the device on the plastic substrate de-bonded.        
For example, this last technique is described in the International patent application published under No. WO 05/050754 on Jun. 2, 2005 in the name of Koninklijke Philips Electronics, in which a substrate arrangement is manufactured by comprising a rigid carrier substrate, like glass, and a plastic substrate over the rigid carrier substrate. To release the plastic substrate from a glass carrier a heating method is used. By heating the glass and the plastic substrate, the plastic substrate and the electronic components formed on the substrate are released from the glass carrier. The release process, also called Electronics on Plastic by Laser Release (EPLaR) as proposed is a laser lift-off process. Laser light at ultraviolet wavelengths is used to cause the lift-off of the plastic substrate from the underlying carrier. It has been also indicated that the release process may be a photo-ablation process due to multiple-photon processes, including localized heating. A suggested material for this process is polyimide, which is chosen for its high-temperature stability and high absorption of UV energy.
The FIGS. 2A-2C show the steps of a manufacturing method comprising a step of releasing the carrier based on the laser ablation technique. More in details, as shown in FIG. 2A, a transparent carrier 10, like quartz, is provided and a liquid plastic precursor 20, such as polyimide, is spun on the carrier 10; the plastic precursor 20 is then cured to form a plastic substrate 30 bonded to the carrier 10. The carrier 10 with the plastic substrate 30 fixed to it is showed in FIG. 2B. At the end, an electronic device 40 is realized over the plastic substrate 30 and then released from the carrier 10 exposing it to a laser light at ultraviolet wavelengths to cause the lift-off.
There are some problems in using a heating effect to lift-off a plastic substrate from the glass. Sufficient energy is needed to enable lift off to occur, but without damaging either the plastic substrate or the components formed, damages occurring for the thermal expansion effects. When using a laser lift-off process, higher wavelengths within the UV spectrum are preferable because lower wavelengths are more absorbed by the glass substrate, making the laser release less effective. For example commercially available lasers which operate at 308 nm or 351 nm are preferred. At these high wavelengths, the energy absorbed in the plastic layer is statistically distributed without complete thermalization in the plastic polymer molecules. This gives rise to localized heating effects, which can in turn result in damages to the plastic substrate or the components mounted on it. This can also results in partial or poor lift-off from the carrier.
Moreover, the transparent carrier substrates, such as quartz, are very expensive.